Download FORMAL CHARGE AND OXIDATION NUMBER - IDC

FORMAL CHARGE AND OXIDATION
NUMBER
Although the total number of valence electrons in a molecule is easily calculated,
there is not aways a simple and unambiguous way of determining how many reside
in a particular bond or as non-bonding pairs on a particular atom. For example, one
can write valid Lewis octet structures for carbon monoxide showing either a
double or triple bond between the two atoms, depending on how many nonbonding
pairs are placed on each: C::O::: and :C:::O: (see Problem Example 3 below).
The choice between structures such as these is usually easy to make on the
principle that the more electronegative atom tends to surround itself with the
greater number of electrons. In cases where the distinction between competing
structures is not all that clear, an arbitrarily-calculated quantity known as
the formal charge can often serve as a guide.
The formal charge on an atom is the electric charge it would have if all
bonding electrons were shared equally with its bonded neighbors.
How to calculate the formal charge on an atom in a molecule
The formal charge on an atom is calculated by the following formula:
In which the core charge is the electric charge the atom would have if all its
valence electrons were removed. In simple cases, the formal charge can be worked
out visually directly from the Lewis structure, as is illustrated farther on.
Problem Example 1
Find the formal charges of all the atoms in the sulfuric acid structure
shown here.
Solution: The atoms here are hydrogen, sulfur, and double- and single-bonded
oxygens. Remember that a double bond is made up of two electron-pairs.
hydrogen: FC = 1 – 0 – 1 = 0
sulfur: FC = 6 – 0 – 6 = 0
hydroxyl oxygen: FC = 6 – 4 – 2 = 0
double-bonded oxygen: FC = 6 – 4 – 2 = 0
Using formal charge to select the best Lewis structure
The general rule for choosing between alternative structures is that the one
involving the smallest formal charges is most favored, although the following
example shows that this is not always the case.
Problem Example 2
Write out some structures for carbon monoxide CO, both those that do and do not
obey the octet rule, and select the "best" on the basis of the formal charges.
Solution:
Structure that obeys the octet rule:
a) For :C:::O: Carbon: 4 – 2 – 3 = –1; Oxygen: 6 – 2 – 3 = +1
Structures that do not obey the octet rule (for carbon):
b) For :C:O::: Carbon: 4 – 2 – 1 = +1; Oxygen: 6 – 6 – 1 = –1
c) For :C::O:: Carbon: 4 – 2 –2 = 0; Oxygen: 6 – 4 – 2 = 0
Comment: All three structures are acceptable (because the formal charges add up
to zero for this neutral molecule) and contribute to the overall structure of carbon
monoxide, although not equally. Both experiment and more advanced models
show that the triple-bonded form (a) predominates.
Formal charge, which is no more than a bookkeeping scheme for electrons, is by
itself unable to predict this fact.
In a species such as the thiocyanate ion SCN— (below) in which two structures
having the same minimal formal charges can be written, we would expect the one
in which the negative charge is on the more electronegative atom to predominate.
The electrons in the structures of the top row are the valence electrons for each
atom; an additional electron (purple) completes the nitrogen octet in this negative
ion. The electrons in the bottom row are divided equally between the bonded
atoms; the difference between these numbers and those above gives the formal
charges.
Formal charge can also help answer the question ―where is the charge located?‖
that is frequently asked about polyatomic ions. Thus by writing out the Lewis
structure for the ammonium ion NH4+, you should be able to convince yourself
that the nitrogen atom has a formal charge of +1 and each of the hydrogens has 0,
so we can say that the positive charge is localized on the central atom.
Oxidation number
This is another arbitrary way of characterizing atoms in molecules. In contrast to
formal charge, in which the electrons in a bond are assumed to be shared
equally, oxidation number is the electric charge an atom would have if the bonding
electrons were assigned exclusively to the more electronegative atom. Oxidation
number serves mainly as a tool for keeping track of electrons in reactions in which
they are exchanged between reactants, and for characterizing the ―combining
power‖ of an atom in a molecule or ion.
The following diagram compares the way electrons are assigned to atoms in
calculating formal charge and oxidation number in carbon monoxide.
Source: http://www.chem1.com/acad/webtext/chembond/cb04.html